Please can we get it to be a valid unit?
It makes no sense to arbitrarily limit what a valid length unit is on
a given element. A length is a length. Why should % not be valid as a
unit on border-width but valid everywhere else?! It was proposed years
ago ( http://lists.w3.org/Archives/Public/www-style/2008Jun/0032.html
) and shot down despite the use-case being stated. At the moment those
of us doing nice % based responsive designs can’t use a border in
them. Not without the math breaking and the layout screwing up
entirely. That’s caught me out on every single responsive design I’ve
done, and I’ve had to adjust the design each time.